The invention relates to a starter circuit comprising a series arrangement of a glowswitch and a safety switch, which glowswitch and safety switch are each provided with an enclosed space, with a pair of current conductors which enter the respective enclosed space. A bimetal element is arranged in the enclosed space and is connected to one of the current conductors, the enclosed space of the glowswitch being provided with an ionizable filling. The glowswitch is open in the cold state, and the safety switch is closed in the cold state, while in addition the starter circuit comprises a heat source for opening the safety switch and a heat source for keeping the safety switch open.
Such a starter circuit is known from DE 815 365. The glowswitch is the heat source for opening the safety switch in the known starter circuit. The glowswitch and the safety switch are thermally coupled to one another for this purpose. A resistor shunting the safety switch acts as a heat source for keeping the safety switch open.
The known starter circuit serves for igniting a low-pressure discharge lamp with preheatable electrodes. The known starter circuit for this purpose forms part of a circuit in which the low-pressure discharge lamp is connected in series with an inductive impedance having connection terminals and in which the starter circuit shunts the discharge lamp and is connected in series with the electrodes.
When the connection terminals are connected to a supply source, the glowswitch alternately enters an open and a closed state. In the closed state, a current flows through the electrodes via the starter circuit. This current brings the electrodes of the discharge lamp to a temperature at which a sufficient degree of electron emission occurs for enabling the discharge lamp to ignite.
A transition from the closed state to the open state of the glowswitch interrupts the current through the electrodes of the lamp, so that the inductive element connected in series with the lamp will generate an ignition voltage pulse. If this ignition voltage pulse effects a discharge between the electrodes of the discharge lamp, the voltage across the discharge lamp, and accordingly the voltage between the current conductors of the glowswitch, drops so strongly that the glowswitch remains in its open state. The contact between the current conductors and the glowswitch remains broken then.
If, however, repeated ignition efforts do not lead to a discharge in the low-pressure discharge lamp, the safety switch is heated owing to the thermal coupling to the glowswitch to such an extent that the bimetal element of the safety switch is bent. The safety switch thus assumes an open position. As a result of this, a current will start to flow through the resistive impedance which shunts the safety switch. The heat generated in this resistive impedance keeps the safety switch in its open state, so that no further ignition pulses are generated. It is a disadvantage that the time elapsing until the moment the safety switch enters a closed state again after switching-off of the supply source, called recovery time hereinafter, is comparatively long. This renders it more difficult to determine whether it is sufficient to replace the lamp or whether there is another defect.